Except at high latitudes, the ocean is divided into three horizontal depth zones based on density: the mixed layer, pycnocline, and deep layer. At high latitudes, the pycnocline and mixed layer are absent.

A cross-sectional longitudinal profile of the Atlantic Ocean from 60
degrees N to 60 degrees S showing the location of the mixed layer,
pycnocline, and deep layer. Note that the ocean (and deep layer)
extend to depths of 4000 to 6000 m.

Wind-driven surface currents are restricted mostly to the ocean's uppermost 100 m (300 ft) layer or so depending upon the depth of the pycnocline. This is because the thickness of the surface mixed layer is typically 100 m or less. The pycnocline acts as a porous boundary that allows some kinetic energy to penetrate into deep water. The strongest currents generally occur in the ocean's surface layer although some surface currents such as boundary currents like the Gulf Stream (discussed later) can be relatively strong to depths of several hundred meters. Surface currents are changeable, continually responding to variations in the wind, precipitation, and heating or cooling. Stirring of surface waters by the wind produces a well-mixed layer of uniform or nearly uniform density. For this reason, the ocean surface is called the mixed layer. We know most about the mixed layer because ships, aircraft, and Earth-orbiting satellites can readily monitor it.

The pycnocline, situated between the mixed layer and the deep layer, is where water density increases rapidly with depth because of changes in temperature and/or salinity. Recall that cold water is denser than warm water and salty water is denser than fresh water. Where a decline in temperature with depth is responsible for the increase in density with depth, the pycnocline is also a thermocline. On the other hand, if an increase in salinity is responsible for the increase in density with depth, the pycnocline is also a halocline. Typically, the pycnocline extends to a depth of 500 to 1000 m (1600 to 3300 ft). (However, in middle latitudes seasonal pycnoclines may develop within the mixed layer.) The dark, cold deep layer below the pycnocline accounts for most of the ocean's mass. Within the deep layer, density increases gradually with depth and water moves slowly; in only a few locations (usually near the bottom) are water movements fast enough to be considered currents.

The ocean's three-layer structure is an example of how gravity separates a fluid into layers such that the density of each layer is less than the density of the layer below it. More dense fluids sink and less dense fluids rise. The ocean's pycnocline is very stable thus suppressing mixing between the mixed layer and deep layer; that is, the pycnocline acts as a barrier to vertical motion within the ocean. The concept of stability is useful in understanding this property of the pycnocline.

Stability as used here refers to vertical motions of ocean water. A system is described as stable if it tends to persist in its original state without changing. Following a disturbance (i.e., vertical motion), a stable system returns to its initial state or condition. As noted above, the usual stable state of the ocean features a layer of water that is warmest near its interface with the atmosphere (the mixed layer) and the mixed layer overlies water that becomes denser with increasing depth (the pycnocline). Strong storm winds may temporarily disturb this stable stratification bringing colder than usual water to the surface. Once the wind slackens, however, the original layered structure is soon restored.

Adapted from DataStreme Ocean and
used with permission of the
American Meteorological Society.